Energy harvesting is a process which captures small amounts of energy that would otherwise be lost as heat, light, sound, vibration or movement. In general, the energy can be stored in a capacitor, super capacitor or battery and the small amounts of energy can be used to power small low-energy electronics such as wireless sensor networks and wearable electronics. The external sources that generate energy include solar power, thermal power, wind power, salinity gradients and kinetic energy.
Radio frequency (RF) energy harvesting can occur to capture RF energy using a power generating circuit linked to a receiving antenna. This RF energy can then be converted into usable DC voltage. In the same way as other energy harvesting sources, the converted power can either be stored directly in a battery, or can be accumulated in a capacitor or supercapacitor to power circuits directly or charge a battery. It is known that the levels of RF energy available drop considerably with distance, following the inverse square law. Devices are typically tuned to operate most efficiently at specific frequencies.
Energy harvesting has been used in the past for powering industrial sensors, however, in these cases both the source and the harvester are fixed in position, allowing for an easier wireless transfer of energy between them. Biological systems, on the other hand, represent an extra challenge due to the fact that the subject has the ability to move freely. These movements create constant alterations in the receiving antenna position, which diminish the ability of harvesting systems to collect energy. Such limitations are amplified by the fact that the subject movements are random and cannot be anticipated in order to create a correction algorithm.
Battery drainage can affect a sensor's performance over time and significantly limit operational lifetime. In the case of biological sensors, which are often implanted, battery replacement can be challenging and often unviable. This creates a need for sensors that can be alternatively powered.